there are six
basic stages in the life cycle of viruses:
Stage 1
Attachment is a specific binding between viral capsid proteins and specific receptors on the host cellular surface. This specificity determines the host range of a virus. For example, HIV infects a limited range of human
leucocytes. This is because its surface protein,
gp120, specifically interacts with the
CD4 molecule – a
chemokine receptor – which is most commonly found on the surface of
CD4+ T-Cells. This mechanism has evolved to favour those viruses that infect only cells in which they are capable of replication. Attachment to the receptor can induce the viral envelope rotein to undergo changes that results in the
fusion of viral and cellular membranes, or changes of non-enveloped virus surface proteins that allow the virus to enter.
Stage 2
Penetration follows attachment: Virions enter the host cell through receptor-mediated
endocytosis or
membrane fusion. This is often called
viral entry. The infection of plant and fungal cells is different from that of animal cells. Plants have a rigid cell wall made of
cellulose, and fungi one of chitin, so most viruses can get inside these cells only after trauma to the cell wall.[sup]
[91][/sup] However, nearly all plant viruses (such as tobacco mosaic virus) can also move directly from cell to cell, in the form of single-stranded nucleoprotein complexes, through pores called
plasmodesmata.[sup]
[92][/sup] Bacteria, like plants, have strong cell walls that a virus must breach to infect the cell. However, given that bacterial cell walls are much less thick than plant cell walls due to their much smaller size, some viruses have evolved mechanisms that inject their genome into the bacterial cell across the cell wall, while the viral capsid remains outside.
Stage 3
Uncoating is a process in which the viral capsid is removed: This may be by degradation by viral
enzymes or host enzymes or by simple dissociation; the end-result is the releasing of the viral genomic nucleic acid.
Stage 4
Replication of viruses involves primarily multiplication of the genome. Replication involves synthesis of viral messenger RNA (mRNA) from "early" genes (with exceptions for positive sense RNA viruses), viral
protein synthesis, possible assembly of viral proteins, then viral genome replication mediated by early or regulatory protein expression. This may be followed, for complex viruses with larger genomes, by one or more further rounds of mRNA synthesis: "late" gene expression is, in general, of structural or virion proteins.
Stage 5
Following the structure-mediated
self-assembly of the virus particles, some modification of the proteins often occurs. In viruses such as HIV, this modification (sometimes called maturation) occurs
after the virus has been released from the host cell.
Stage 6
Viruses can be
released from the host cell by
lysis, a process that kills the cell by bursting its membrane and cell wall if present: This is a feature of many bacterial and some animal viruses. Some viruses undergo a
lysogenic cycle where the viral genome is incorporated by
genetic recombination into a specific place in the host's chromosome. The viral genome is then known as a "
provirus" or, in the case of bacteriophages a "
prophage".[sup]
[95][/sup] Whenever the host divides, the viral genome is also replicated. The viral genome is mostly silent within the host; however, at some point, the provirus or prophage may give rise to active virus, which may lyse the host cells.[sup]
[96][/sup] Enveloped viruses (e.g., HIV) typically are released from the host cell by
budding. During this process the virus acquires its envelope, which is a modified piece of the host's plasma or other, internal membrane.
